Double Free in Dynamodb

Double free is a memory‑safety bug that occurs when a program calls free() (or the language‑specific equivalent) more than once on the same memory address. In the context of DynamoDB, this issue typically appears in application code that uses the AWS SDK for C/C++ (or a custom low‑level HTTP client) to interact with the DynamoDB data plane and then manually manages buffers that hold attribute values.

Consider a common pattern where a developer retrieves a string attribute from a DynamoDB item, copies it into a manually allocated buffer, and later attempts to free that buffer:

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void GetUserId(const Aws::String& tableName, const Aws::String& userId) {
    Aws::DynamoDB::DynamoDBClient client;
    Aws::DynamoDB::Model::GetItemRequest request;
    request.SetTableName(tableName);
    request.AddKey("userId", Aws::DynamoDB::Model::AttributeValue().SetS(userId));

    auto outcome = client.GetItem(request);
    if (!outcome.IsSuccess()) {
        return; // error handling omitted for brevity
    }

    const Aws::DynamoDB::Model::AttributeValue& attr = outcome.GetResult().GetItem().at("userId");
    const char* rawValue = attr.GetS().c_str();

    // BUG: manually allocate a copy and then free it twice
    char* buffer = static_cast(std::malloc(rawValue ? std::strlen(rawValue) + 1 : 1));
    if (rawValue) std::strcpy(buffer, rawValue);
    else buffer[0] = '\0';

    // ... use buffer ...

    std::free(buffer); // first free
    // Later, mistakenly freeing again (e.g., in a cleanup routine)
    std::free(buffer); // double free -> undefined behavior, possible crash
}

The bug is not in DynamoDB itself; it resides in the client code that mishandles memory returned by the SDK. Similar patterns can appear when using the AWS C SDK (aws-c-common) or when building a custom HTTP client that parses DynamoDB JSON responses with malloc/free and fails to track ownership correctly.

An attacker who can trigger the vulnerable code path (e.g., by sending a request that causes the application to allocate a buffer for a large attribute value) may be able to cause a crash or, in some environments, achieve arbitrary code execution by exploiting the heap corruption that follows a double free.

Because double free manifests as a runtime memory error, the observable symptom is often an HTTP 500 Internal Server Error returned by the application, sometimes accompanied by a stack trace or a message such as "double free or corruption" or "malloc(): memory corruption". middleBrick’s unauthenticated black‑box scan can surface these error responses when probing the API endpoint.

To detect the issue with middleBrick:

1. Run a scan against the target DynamoDB‑enabled endpoint: middlebrick scan https://api.example.com/items
2. Examine the generated report for findings under the "Internal Server Error" or "Unexpected Response" categories.
3. Look for response bodies that contain keywords like "double free", "corruption", "heap", or "SIGABRT". Such strings indicate that the underlying process encountered a memory‑safety fault.
4. Correlate the finding with the specific API operation (e.g., GET /items/{id}) to pinpoint which client‑side code path is being exercised.

middleBrick does not instrument the application or access its source code; it relies on the observable behavior of the endpoint. If the application hides internal errors and returns generic 500 pages without detail, the double free may not be directly visible. In those cases, enabling detailed logging in the application (e.g., AWS X‑Ray, CloudWatch Logs) and reviewing crash dumps or core files is necessary to confirm the bug.

Note: middleBrick’s scanning is limited to the unauthenticated attack surface; it cannot detect defects that only appear after authentication or behind complex business logic unless those paths are reachable without credentials.

The correct remediation is to let the AWS SDK manage memory for you and avoid manual free/malloc pairs on data obtained from DynamoDB responses. The SDK’s string and buffer types (Aws::String, Aws::Utils::ByteBuffer) follow RAII semantics, automatically releasing memory when they go out of scope.

Revised version of the earlier example:

#include 
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void GetUserIdSafe(const Aws::String& tableName, const Aws::String& userId) {
    Aws::DynamoDB::DynamoDBClient client;
    Aws::DynamoDB::Model::GetItemRequest request;
    request.SetTableName(tableName);
    request.AddKey("userId", Aws::DynamoDB::Model::AttributeValue().SetS(userId));

    auto outcome = client.GetItem(request);
    if (!outcome.IsSuccess()) {
        // handle error appropriately
        return;
    }

    const Aws::DynamoDB::Model::AttributeValue& attr = outcome.GetResult().GetItem().at("userId");
    // No manual allocation needed; use the SDK string directly
    const Aws::String& userIdValue = attr.GetS();

    // Use userIdValue as needed; it will be cleaned up automatically
    // Example: log or return the value
    // std::cout << "User ID: " << userIdValue << std::endl;
}

If you must work with a raw C‑style buffer (e.g., when interfacing with a legacy library), follow these rules:

• Allocate memory with malloc/calloc or new.
• Ensure every allocation has exactly one corresponding free/delete.
• Set the pointer to nullptr after freeing to prevent accidental reuse.
• Consider using smart pointers (std::unique_ptr with a custom deleter) or std::string to encapsulate ownership.